This invention relates to a varifocal lens device. More particularly, this invention relates to a varifocal lens device provided with a variable power optical system composed of a variable power lens group and a focusing lens group disposed in an optical axis, for compensating a deviation produced in the image forming position for an object when the focusing lens group of the optical system is set at a focusing position between the nearest possible position to the infinitely far position on the optical axis corresponding to the object distance falling between the shortest possible distance to the infinitely long distance and the overall focal length of the variable power optical system is subsequently renewed from a first to a second focal length selected freely between the shortest possible focal length to the longest possible focal length by means of the variable power lens group.
A zoom lens does not incur any deviation in the image forming position (so-called unstable focusing or incorrect focusing) from a zooming work and, therefore, is free from the necessity for adjusting a focus after each zooming work and excellent in operability. Since it nevertheless has a large F-number for the diaphragm as compared with the monofocal lens, it requires the user of the zoom lens mounted in a single lens reflex camera, for example, to possess a considerable amount of skill in the adjustment of a focus (focusing) with a range finder. In recent years, the automatic camera focusing mechanisms have advanced to a point where the problem of difficult focusing mentioned above is solved and the cameras using zoom lenses are enabled to manifest fully the mobility inherent in the zoom lens. The zoom lenses enjoy notably improved operability and, therefore, the users thereof are enabled to devote all their attention solely to the selection of a composition of a given scene to conform to their design in mind.
Generally, the focusing of the zoom lens is effected by the movement of a focusing lens group which is disposed in part of a variable power optical system. The zoom lens has an advantage that the amount of movement of the focusing lens group for object distance is substantially constant throughout the entire zone of zooming (hereinafter this fact will be referred to as "equivalent movement"). For the operation of the zoom lens, therefore, it suffices to attach a graduation of the distance of object to a moving member (distance ring) for the focusing lens group and inscribe an index on a stationary ring juxtaposed to the moving member. Again it has an advantage that it does not require to change the graduation of the object distance, depending on the particular zooming work to be involved.
An attempt at optically designing a zoom lens on the conditions realizing the equivalent movement mentioned above entails a disadvantage that the lens composition notably gains in complexity, though to a varying degree depending on the particular lens composition in the variable power optical system. This zoom lens has another disadvantage that the amount of movement (amount of delivery) of the focusing lens group is unduly large on the wide-angle side. This fact entails a drawback that the zoom lens inevitably has a large outside diameter and the lens itself and the lens barrel weigh heavily.
For the solution of these problems, the varifocal lens which is relieved of the requirement for the equivalent movement mentioned above has been proposed. This varifocal lens, however, has a drawback that it generates a deviation in the image forming position when it is subjected to the work of power variation corresponding to the work of zooming of the zoom lens mentioned above. It is conceivably possible to solve this problem by having part of the variable power optical system constituting the varifocal lens as a focusing lens group adapted to generate a focusing movement independently of the work of power variation mentioned above, determining by computation the deviation (hereinafter referred to occasionally as "shift") of the image forming position resulting from the work of power variation, compensating the position of the focusing lens group based on the outcome of the computation, and thereby enabling the varifocal lens to acquire substantially the same operability as the zoom lens.
The automatic compensation of shift may be possibly realized by constructing the so-called analog-servo mechanism for analog control with a variable power driving section serving to drive the variable power optical system as with a variable power motor for setting a desired focal length, a focus driving section serving to drive the focusing lens group as with a focus motor, a focal length detecting section such as, for example, a potentiometer serving to detect the currently set focal length as in the form of absolute value of voltage, and a focus position detecting section such as, for example, a potentiometer capable of detecting the current position of the focusing lens group as in the form of absolute value of voltage.
The varifocal lens described above, however, cannot be easily realized with the analog-servo mechanism (analog computation) because the focusing position of the focusing lens group for an object varies with the focal length and the locus of this variation describes a hyperbola having the focal distance (output of the focal length detector) as the variable thereof. If the varifocal lens is realized with this mechanism at all, it will be extremely complicated in construction and will be expensive as an inevitable consequence.
When the driving motor for driving the focusing lens group is adapted for analog control, it is liable to induce the phenomenon of hunting unless it makes use of an expensive and highly accurate circuit member. It is, therefore, difficult to attain any improvement in the speed of the analog control of the driving motor.
In recent years, the automation of camera manipulation has advanced to a point where microcomputers find extensive utility for comprehensive efficient control of various complicated automatic works involved. In a camera using a microcomputer, not only the center of control but also peripheral components are designed for digital operation. For example, the data on distance measurement produced from the distance measuring section, an essential component for the automatic focusing device, are also based on digital notation. It is, therefore, possible of course to provide the varifocal lens itself with the automatic focusing device mentioned above. Economically it is advantageous to utilize the automatic focusing device already incorporated in the camera concurrently for automatic focusing of the varifocal lens. When the existing automatic focusing device is to be concurrently used as described above where the control system for the varifocal lens happens to be designed for analog processing, however, the interface between the photographic lens and the camera body is so complicated as to constitute itself a problem.
Even when the control system for the varifocal lens is adapted for digital processing, the following problem remains to be solved. When the work of power variation is to be made while the focusing lens group is in the focused position, for example, the control system for the varifocal lens tends to retain the focused state by moving the focusing lens group along the hyperbola mentioned above. Generally for purpose of spontaneously smoothening the change of the angle of view in the finder, for example, the focusing lens group is allowed to reach the focal length to be set by alternately repeating the focusing motion for compensation (hereinafter referred to as "shift compensating motion") and the motion of power variation for changing focal length. Whenever the alternately repeated shift motion is performed, the control system requires to compute the amount of delivery of the focusing lens group to the focusing position to be subsequently made. The standard value which forms the basis for the computation of the amount of delivery, however, is renewed for each computation to entail a disadvantage that the errors in the detection of the positions of the variable power lens group and the focusing lens group, the errors in the computation, the errors in the shift control, and other similar errors are suffered to accumulate gradually at a proportional sacrifice of the accuracy with which the shift control is effected.
Where the output of the focal distance detector is subjected to A/D conversion with an A/D converter and the outcome of the conversion is used as the information on focal length for the purpose of facilitating the interface between the photographing lens and the camera body, the output of the A/D converter always fluctuates though very minutely even if the variable power lens group is kept fixed on the optical axis. Further in the computation of the amount of delivery for the shift compensation on the basis of the output of the A/D converter, the computation errors such as those involved in rounding operations occur to aggravate the fluctuation of the output. When the focusing lens group is directly driven with the outcome of the computation, namely the continuous control is effected after the pattern of the analog control, the fluctuation mentioned above causes the focusing lens group to oscillate as an inevitable consequence.
In the varifocal lens of the type in which the focusing position for an object at an infinite distance is invariable relative to the change in the focal length, the inclination of the hyperbola reaches the maximum at the nearest possible position. When the motion for power variation inclusive of the shift compensating motion mentioned above is carried out from the shortest focal length to the longest focal length, therefore, there is a disadvantage that the time required for switching the shortest focal length to the longest focal length is notably varied.
The driving of the focusing lens group for the sake of the shift compensating motion is effected with a focus motor. Since the direction in which the work of power variation is performed is at the mercy of the user's discretion, the direction in which the focus motor is driven cannot be generally fixed. Furthermore, it takes long time for deciding the driving direction of the focus motor. The varifocal lens of this type, therefore, is deficient in response characteristic. As a solution for this problem, Japanese Patent Laid-Open Publication No. SHO 60(1985)-211,414 discloses an automatic focusing device which is designed to detect the change in the focal length of the variable power lens group and decide the driving direction of the focusing lens group on the basis of the outcome of the detection. In this conventional device, a variable resistor interlocked with a variable power lens group is incorporated so that the direction of power variation of the variable power lens group is detected by subjecting the output voltage of the variable resistor to analog processing and consequently the driving direction of the focusing lens is finally decided. The analog processing is effected by the use of a differential amplifying circuit, a sample hold circuit, and a zero-cross comparator, for example. On the assumption that V.sub.1 stands for the output voltage mentioned above which exists after the previous renewal of focal length and V.sub.2 for the output voltage which is obtained in the current work of power variation, the sample hold circuit retains the output voltage V.sub.1, the differential amplifying circuit produces as the output thereof the difference between the output voltage V.sub.1 retained as described above and the currently produced output voltage V.sub.2, and the zero-cross comparator judges whether the output of the differential amplifying circuit is of a plus quantity or a minus quantity and, based on the output level, decides the driving direction of the focusing lens group.
This conventional device, however, has a possibility of being affected as by the variation of power source voltage applied to the variable resistor, the resolving power of the variable resistor, and the accuracy of the analog processing and consequently compelled to generate an erroneous operation of driving the focusing lens group in the direction opposite the direction to be correctly selected. The situation of this nature entails the possibility that the logic which is operating in the logical circuit for deciding the rotating direction of the focus motor will get out of order under the influence of the outputs of the limiter switches for terminal detection disposed one each at the opposite terminals of the nearest position and the infinite position and the output of the zero-cross comparator and, as the result, the focusing lens group will be deprived of controllability as held in a state of collision against the limiter switch.
Further the fact that the driving direction is produced correctly at times and incorrectly at other times leads to a disadvantage that the focusing lens group is compelled to generate oscillation. As the result, the focus motor incurs wasteful consumption of electricity or the members of the focus driving section undergo unnecessary wear.
In the description given so far, the motion of shift compensation has been portrayed as alternately repeating the motion of power variation and the motion of shift compensation for the sake of easy understanding. When the device is adapted for these two motions to proceed simultaneously, it permits a decrease in the time required for the operation and enjoys an improvement in operational efficiency. Where the varifocal lens operates by itself or by the use of part of the control system of the camera body, the variable power motor and the focus motor both rely for supply of power upon a battery and, for the control system which constitutes itself a load of the battery, the two motors mentioned above are a typical heavy load. When the two motors are simultaneously operated, therefore, the power source incurs a change (drop) of voltage and entails a disadvantage that the operating speed is lowered and the accuracy of positional control of the focusing lens group and the variable power lens group is impaired. The varifocal lens is generally composed of a number of lens groups and the individual lens groups have their motions controlled by means of cam grooves incised in a cam frame and these cam grooves have mutually different shapes. When the focal length is changed from the shortest level to the longest level, for example, the varifocal lens has a disadvantage that it fails to produce the motion of power variation at a uniform speed because the load as viewed from the standpoint of the variable power motor (the torque required for the change mentioned above) is never uniform but is largely varied by the position to which the variable power lens group is moved. From this disadvantage is further derived a drawback that the change of the angle of view is uniform relative to the motion of power variation and the scene produced in the finder has poor appearance particularly when the motion of power variation and the motion of shift compensation are alternately repeated. In other words, this drawback resides in impairment of the accuracy of control. Even when the motion of power variation and the motion of shift compensation are allowed to proceed simultaneously, there is the possibility that the load will be lightened instantaneously and the variable power lens group will be moved with a long quick stroke and, as the result, the shift compensation will exceed the allowable range and, in the worst case, go out of control.